Double HBT base metal micro-bridge

ABSTRACT

A heterojunction bipolar transistor (HBT) device structure is provided which facilitates the reduction of the base-collector capacitance and a method for making the same. The base-collector capacitance is decreased by fabricating a base micro-bridge connecting a base contact to a base mesa on the HBT. The base micro-bridge is oriented along about one of 001, 010, 00{overscore (1)}, and 0{overscore (1)}0 direction to a major flat of the wafer. The HBT device employs a phosphorous based collector material. During removal of the phosphorous based collector material, the base layer is undercut forming the micro-bridge, successfully removing the collector and sub-collector material below the bridge due to the orientation of the micro-bridge. The removal of collector and sub-collector material reduces the base-collector junction area, and therefore reduce the base-collector junction capacitance.

TECHNICAL FIELD

The present invention relates generally to electrical circuits, and moreparticularly to fabrication of heterojunction bipolar transistors.

BACKGROUND OF THE INVENTION

Heterojunction bipolar transistors (HBTs) are widely used in high speedand high frequency applications. The heterojunction bipolar transistor(HBT) offers much higher speeds of operation than the more prevalentmetal-oxide-semiconductor field-effect transistors (MOSFETS) or evenconventional homojunction bipolar transistors, such as npn or pnpsilicon transistors. The HBT offers an alternative technology to metalsemiconductor field effect transistors (MESFETs) and high electronmobility transistors (HEMTs) when a high degree of linearity isdesirable. The use of different materials of differing bandgaps for thecollector, base and emitter provides for additional design flexibility.The HBT is a layered structure that includes a semiconductor substrate,a subcollector, a collector, a base and an emitter stacked one on topthe other in an integral assembly. Metal contacts are formed to connectpower and other circuitry to the emitter, the base and the subcollector.The largest limitation to the operational frequency and speed of the HBTis the base-collector capacitance. The base-collector capacitance islargely due to the collector-base interface area. Reduction of the basearea can introduce higher base resistance due to the reduction of thearea of the base contact.

A common HBT technology is based on Gallium Arsenide (GaAs)/GalliumAluminum Arsenide (GaAlAs) based family. The GaAs based HBT suffers frombase-collector capacitance due to the area of the interface between thebase and collector layers. One method of reducing the base collectorcapacitance of a GaAs based HBT is to implant protons into an area ofthe collector surrounding the emitter so as to electrically insulate theimplanted area. Another HBT technology is based on the Indium Phosphide(InP)/Indium Gallium Arsenide (InGaAs) material family. The InP/InGaAsHBT also suffers from base-collector capacitance. However, protonimplantation has proved ineffective in rendering InP to be sufficientlyinsulating or semi-insulating.

Miyamoto et al. addresses this problem by performing a selective etchantto substantially etch the collector layer under the base layer so as toundercut the edges of the base layer of InP-based HBTs in “Reduction ofBase-Collector Capacitance by Undercutting the Collector andSubcollector in GaInAsInP DHBT's,” IEEE Electron Device Letters, vol.17, March 1996, pp. 97-99. The undercuts are then backfilled withpolyimide to provide mechanical integrity. The reduced size of thecollector, together with the lower dielectric constant of the polyimide,reduces the base-collector capacitance. However, this approach does notreduce the base-collector capacitance component from under the basecontact. Other methods to reduce base-collector capacitance includeusing an electrically insulating region of iron (Fe) doped InP replacingpart of the collector to reduce the base-collector area.

Another method of reducing the base-collector capacitance has been toreduce the base area and form a base metal micro-bridge to a basecontact disposed away from the active portion of the HBT. The base metalmicro-bridge is typically formed by depositing the base metal and thenetching away the semiconductor from underneath a portion of the basemetal. Song et al. demonstrate this technique for an HBT with an InGaAscollector layer in “Reduction of Extrinsic Base-Collector Capacitance inInP/InGaAs SHBTs Using a New Base Pad Design, INP and Related MaterialsConference, 2002, pp. 165-168.” However, it is difficult to fabricatethe base metal micro-bridge for HBTs that have phosphorus basedcollector layers and arsenic based base layers due to the unusualetching characteristics of phosphorous based materials. A similartechnique is demonstrated for removing InP subcollector layers (but notthe collector layer) using a cumbersome double base contact pad by Idaet al. in “InP/InGaAs DHBTs with 341-GHz for a big current density over800 kA/cm², IEEE Electron Device Meeting, 2001, pp. 35.4.1-35.4.4.”

SUMMARY OF THE INVENTION

The following presents a simplified summary of the invention in order toprovide a basic understanding of some aspects of the invention. Thissummary is not an extensive overview of the invention. It is intendedneither to identify key or critical elements of the invention nordelineate the scope of the invention. Its sole purpose is to presentsome concepts of the invention in a simplified form as a prelude to themore detailed description that is presented later.

The present invention relates to a HBT device structure whichfacilitates mitigation of base-collector capacitance and a method formaking the same. The HBT device structure of the present inventionemploys a phosphorous based collector material. During removal of thephosphorous based collector material, for example, using a wet etch,only facets or geometric surfaces that are part of a convex side of theHBT, and are oriented at about a +/−45° (010 or 001 direction) or abouta +/−135° (00{overscore (1)} or 0{overscore (1)}0 direction) from themajor flat of the wafer will be undercut (major flat is in the00{overscore (1)} crystal plane). The base-collector capacitance isdecreased by reducing the base mesa area and fabricating a baseconnecting bridge or micro-bridge that electrically connects a basecontact to the base mesa. The base connecting bridge is positioned alonga 45° angle to a major flat of the wafer. During collector removal, thebase layer is undercut forming the connecting bridge, successfullyremoving the collector and sub-collector material below the bridge. Theremoval of collector and sub-collector material will reduce thebase-collector junction area, and therefore reduce the base-collectorjunction capacitance.

Additionally, a base undercut cantilever base contact structure furtherreduces the base-collector capacitance. The base undercut cantileverbase contact structure is provided by aligning the orientation of theintrinsic HBT portion and base contact portions along a 45° angle to themajor flat. Therefore, undercutting portions of the collector andsubcollector that have a convex geometric surface under the base mesasduring the etching process a cantilever base contact structure.

To the accomplishment of the foregoing and related ends, certainillustrative aspects of the invention are described herein in connectionwith the following description and the annexed drawings. These aspectsare indicative, however, of but a few of the various ways in which theprinciples of the invention may be employed and the present invention isintended to include all such aspects and their equivalents. Otheradvantages and novel features of the invention will become apparent fromthe following detailed description of the invention when considered inconjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic cross-sectional view of a HBT devicestructure in accordance with an aspect of the present invention.

FIG. 2 illustrates a top representative view of a layout of a HBT devicestructure having a 45° angle bridge orientation in accordance with anaspect of the present invention.

FIG. 3 illustrates a top representative view of a layout of a pluralityof HBT device structures on an integrated circuit in accordance with anaspect of the present invention.

FIG. 4 is a schematic cross-sectional illustration of a plurality ofstacked layers for forming a HBT device structure in accordance with anaspect of the present invention.

FIG. 5 is a schematic cross-sectional illustration of the structure ofFIG. 4 undergoing an etch to form an emitter in accordance with anaspect of the present invention.

FIG. 6 is a schematic cross-sectional illustration of the structure ofFIG. 5 after the etch to form the emitter is substantially complete inaccordance with an aspect of the present invention.

FIG. 7 is a schematic cross-sectional illustration of the structure ofFIG. 6 after undergoing a deposition process to form a base contact, aconnecting bridge and a contact to a base mesa on the emitter portion ofthe HBT device structure in accordance with an aspect of the presentinvention.

FIG. 8 is a schematic cross-sectional illustration of the structure ofFIG. 7 after a photoresist layer has been formed on the structure inaccordance with an aspect of the present invention.

FIG. 9 is a schematic cross-sectional illustration of the structure ofFIG. 8 undergoing patterning of the photoresist layer in accordance withan aspect of the present invention.

FIG. 10 illustrates a top representative view of the structure of FIG. 9in accordance with an aspect of the present invention.

FIG. 11 is a schematic cross-sectional illustration of the structure ofFIG. 10 undergoing an etch to form base mesas in accordance with anaspect of the present invention.

FIG. 12 is a schematic cross-sectional illustration of the structure ofFIG. 11 after the etch to form the base mesas is substantially completein accordance with an aspect of the present invention.

FIG. 13 is a schematic cross-sectional illustration of the structure ofFIG. 12 undergoing an etch to form the collector regions and to removecollector material from underneath the connecting bridge in accordancewith an aspect of the present invention.

FIG. 14 is a schematic cross-sectional illustration of the structure ofFIG. 13 after the etch to form the collector regions and to removecollector material from underneath the connecting bridge issubstantially complete in accordance with an aspect of the presentinvention.

FIG. 15 is a schematic cross-sectional illustration of the structure ofFIG. 14 after undergoing a strip of excess photoresist in accordancewith an aspect of the present invention.

FIG. 16 is a schematic cross-sectional illustration of the structure ofFIG. 15 after providing a second patterned photoresist layer andundergoing an etch to form the subcollector regions to removesubcollector material from under the connecting bridge in accordancewith an aspect of the present invention.

FIG. 17 is a schematic cross-sectional illustration of the structure ofFIG. 16 after the etch to form the subcollector regions after undergoinga strip of excess photoresist and to remove subcollector material fromunderneath the connecting bridge and in accordance with an aspect of thepresent invention.

FIG. 18 illustrates a top representative view of the structure of FIG.17 in accordance with an aspect of the present invention.

DETAILED DESCRIPTION OF INVENTION

The present invention relates to a HBT device structure whichfacilitates mitigation of base-collector capacitance and a method formaking the same. The HBT device of the present invention exhibits fasterperformance at higher frequencies than many conventional HBT devices.The HBT device of the present invention employs a phosphorous (e.g.,Indium Phosphide (InP), Indium Gallium Phosphide (InGaP)) basedcollector material. During removal of the phosphorous based collectormaterial, for example, by using a wet etch, only facets that are part ofa convex side of the HBT, and are oriented about a +/−45° (010 or 001direction) or about a +/−135° (00{overscore (1)} or 0{overscore (1)}0direction) from the major flat of the wafer will be undercut (major flatis in the 00{overscore (1)} crystal plane). The base-collectorcapacitance is decreased by fabricating a base micro-bridge on anInP-based HBT or double HBT (DHBT) by positioning the base micro-bridgealong about a +/−45° or about a +/−135° angle to a major flat of thewafer. The HBT has one material for the emitter with another materialfor the collector, while the DHBT has three different materials for theemitter, base and collector. During collector removal, the collectorlayer is undercut forming the micro-bridge, successfully removing thecollector and sub-collector material below the bridge. The removal ofcollector and sub-collector material will reduce the base-collectorjunction area, and therefore reduce the base-collector junctioncapacitance. Additionally, a base undercut cantilever base contactstructure further reduces the base-collector capacitance.

FIG. 1 illustrates a schematic cross-sectional illustration of a HBTdevice structure 10 (e.g., double HBT device) in accordance with thepresent invention. The device structure 10 includes an InP substratewafer 12. The InP substrate 12 provides mechanical support for thedevice structure 10, and is of a thickness suitable for providing suchsupport. The device structure 10 includes a base contact portion 11 andan intrinsic HBT portion 13. The base contact portion 11 includes afirst subcollector region 14, a first collector region 16 residing overthe first subcollector region 14, a first base mesa 22 residing over thefirst collector region 16 and a base contact 28 residing over the firstbase mesa 22. The first subcollector region 14, the first collectorregion 16 and the first base mesa 22 are provided for mechanical supportof the base contact 28 and are not provided to form electrically activecomponents of the HBT device structure 10. The intrinsic HBT portion 13includes a second subcollector region 15 residing over the InP substrate12, a second collector region 20 residing over the second subcollectorregion 15, a second base mesa 26 residing over the second collectorregion 20 and an emitter 32 residing over the second base mesa 26. Thesecond subcollector region 15, the second collector region 20, thesecond base mesa 26, and the emitter 32 form the electrically activecomponents of the total HBT device structure 10.

The base material that forms the first and second base mesas 22 and 26is comprised of an arsenic based material (e.g., Indium Gallium Arsenide(InGaAs), Indium Aluminum Arsenide (InAlAs), Indium Aluminum GalliumArsenide (InAlGaAs), Gallium Arsenide Antimonide (GaAsSb), GalliumArsenide (GaAs)). The collector material that forms the collectorregions 16 and 20 and the subcollector regions are comprised of aphosphorous based material (e.g., Indium Phosphide (InP), Indium GalliumPhosphide (InGaP)). While arsenic based materials undercut easily duringa selective etching process, the phosphorous based materials do notreadily undercut many geometric structures or facets. The presentinvention provides a mechanism to undercut a phosphorous layer byorienting a conductive bridge 30 (e.g., metal bridge) that connects thesecond base mesa 26 to a base contact 28 along about a +/−45° or about a+/−135° angle to the major flat of the substrate 12. The conductivebridge 30 can be formed from a variety of conductive materials (e.g.,metal).

An open area 18 resides below a conductive bridge 30 and a remainingbase material portion 24. Alternatively, the remaining base materialportion 24 can be removed during fabrication. The conductive bridge 30forms an electrical connection from the base mesa 26 to the base contact28. Therefore, the base mesa area 26 can be reduced in area mitigatingthe effects of the base-collector capacitance of the HBT devicestructure 10. The conductive bridge 30 is oriented at about a +/−45° orabout a +/−135° angle from the major flat of the substrate 12.Therefore, during the etching of the collector and the subcollectormaterial, the portion of the collector and the subcollector underneaththe base material portion 24 and the conductive bridge 30 is undercut onany convex geometric side or facet of the HBT device structure 10.

FIG. 2 illustrates a top representative view of a layout of a HBT 40having a 45° angle bridge orientation in accordance with an aspect ofthe present invention. The HBT 40 includes a base contact portion 41having a first longitudinal axis B-B and an intrinsic HBT portion 43having a second longitudinal axis A-A substantially normal to the firstlongitudinal axis. The base contact portion 41 and the intrinsic HBTportion 43 are coupled to one another by a conductive bridge 48 orientedat about a +/−45° or about a +/−135° angle with respect to the majorflat of the wafer. The base contact portion 41 includes a base contact42 and a first base/collector mesa 46 illustrated in dashed lines. Theintrinsic HBT portion 43 includes an emitter 52 and a secondbase/collector mesa 56 illustrated in dashed lines. A photoresist ornitride hardmask 44 is employed during formation of the first and secondbase/collector mesas 46 and 56. A conductive contact portion 50 couplesa base mesa portion of the second base/collector mesa 56 to the basecontact 42 via the conductive bridge 48. A collector contact 60 iscoupled to a subcollector region 68 of the HBT 40.

During a collector etch process (e.g., selective collector wet etch),the collector material is removed and undercuts underneath the basematerial below the bridge in the direction of the arrows 58. Since thecollector material is formed from a phosphorous based material, thecollector etch process only undercuts facets that are oriented at abouta +/−45° or about a +/−135° to a major flat of the wafer. This isreferred to as the [010] family of planes. Additionally, the collectoretch only undercuts planes that are convex (i.e., greater or equal to180°) not concave planes (i.e., less than 180°). Therefore, thecollector etch will undercut the convex planes 62 in the direction ofarrows 58, but not the concave planes 64. The subcollector material isalso formed from a phosphorous based material. A sub-collector etch isthen performed to form the subcollector region 68, and remove thesubcollector material from underneath the conductive bridge 48.

FIG. 3 illustrates a top representative view of a layout of a pluralityof HBTs on an integrated circuit 80 disposed on a wafer with each HBThaving about a +/−45° or about a +/−135° angle bridge orientation inaccordance with another aspect of the present invention. The HBTs have abase undercut cantilever base contact structure that further reduces thebase-collector capacitance. The base undercut cantilever base contactstructure is provided due to alignment of the orientation of theintrinsic HBT portions and base contact portions along about a +/45° orabout +/−135° angle to a major flat. Therefore, portions of thecollector and subcollector that have a convex geometric surface underthe base mesas will undercut during the etching process. Additionally,aligning the intrinsic HBT portions and base contact portions in thismanner facilitates improved density packing of the HBT devices on theintegrated circuit 80. In the illustration of FIG. 3, a first HBT device82 resides next to a second HBT device 110. The second HBT device 110illustrates an alternate geometry from the first HBT device 82. However,the components of the first HBT device 82 and the second HBT device 110are identical, therefore, a general discussion of the second HBT device110 will be omitted.

The first HBT device 82 includes a base contact portion 81 and anintrinsic HBT portion 83 aligned along about a +/−45° or about a +/−135°angle with respect to a major flat of the wafer. The base contactportion 81 and the intrinsic HBT portion 83 are coupled to one anotherby a conductive bridge 90 oriented at about a +/450 or about +/−135°angle with respect to the major flat of the wafer, and aligned with thebase contact portion 81 and the intrinsic HBT portion 83. The basecontact portion 81 includes a base contact 86 and a first collector mesa84 illustrated in dashed lines. The intrinsic HBT portion 83 includes anemitter 94 and a second collector mesa 98 illustrated in dashed lines. Aphotoresist or dielectric hardmask 88 is employed during formation ofthe first and second collector mesas 84 and 98. A first base mesa 87 anda second mesa 89 reside underneath the dielectric hardmask 88. Aconductive contact portion 92 couples the second base mesa 89 to thebase contact 86 via the conductive bridge 90. A collector contact 104 iscoupled to a subcollector region 102 of the HBT 82.

During a collector etch process (e.g., selective collector wet etch),the collector material is removed and undercuts the base material belowthe bridge in the direction of the arrows 100. Since the collectormaterial is formed from a phosphorous based material, the collector etchprocess only undercuts facets that are oriented at about a +/−45° orabout a +/−135° to a major flat of the wafer. Additionally, thecollector etch only undercuts planes that are convex (i.e., greater orequal to 180°) not concave planes (i.e., less than 180°). Therefore, thecollector etch will undercut the convex planes 104 in the direction ofthe arrows 100, but not the concave planes 106. During the collectoretch process, the collector material will also be removed fromunderneath the bridge of the second HBT 110 in the direction of thearrows 112. It is to be appreciated that any number of HBTs can befabricated concurrently, only limited by the size of the HBTs, theintegrated circuits and the wafer on which the integrated circuitsreside.

Turning now to FIGS. 4-18, process blocks in connection with fabricationof the HBT device in accordance with an aspect of the present inventionare described. A substrate 120 (e.g., InP substrate, GaAs substrate) orwafer is provided with several stacked layers disposed above thesubstrate 120. A subcollector layer 122 resides over the substrate 120,a collector layer 124 overlays the subcollector layer 122, a base layer126 overlays the collector layer 124 and an emitter layer 128 overlaysthe base layer 126. The subcollector layer 122, the collector layer 124,the base layer 126 and the emitter layer 128 can be formed by epitaxialgrowth of each layer. It is to be appreciated that any suitabletechnique for forming the various layers can be employed such asMolecular Beam Epitaxy (MBE), Metal Organic Chemical Vapor Deposition(MOCVD) and Chemical Beam Epitaxy (CBE). It is to be appreciated thatother layers can be added such as emitter caps, etch stops and gradinglayers without appreciably modifying the fabrication of the HBT device.

The base layer 126 is formed from an arsenic based material (e.g.,InGaAs, InAlAs, InAlGaAs, GaAsSb, GaAs). The collector layer 124 and thesubcollector layer 122 are formed from a phosphorous based material(e.g., InP, InGaP). Alternatively, the subcollector can be formed froman arsenic based material. As previously discussed, arsenic basedmaterials undercut easily during a selective etching process, while thephoshphorous based materials do not readily undercut many geometricstructures. Therefore, to perform and undercut operation to remove thecollector material and subcollector material beneath a conductive bridgeconnecting a base contact portion with an intrinsic HBT portion, theconductive bridge will be oriented along a 45° angle to a major flat ofthe substrate 120.

An etch 200 (e.g., anisotropic reactive ion etching (RIE), wet etch)(FIG. 5) is performed to remove the emitter material layer 128 employinga photoresist or metal mask 129 to form an emitter 130 (FIG. 6). Anysuitable etch technique may be used to etch the emitter material layer128. For example, the emitter material layer 128 can be anisotropicallyetched with a plasma gas(es), herein carbon tetrafloride (CF₄)containing fluorine ions, in a commercially available etcher, such as aparallel plate RIE apparatus or, alternatively, an electron cyclotronresonance (ECR) plasma reactor by a wet etch or a by a combination ofthese techniques to form the emitter 130.

Next, a deposition is performed on the structure of FIG. 6. Thedeposition includes depositing a base metal (e.g., aluminum, titanium,gold, tungsten, platinum) to provide a connection between the base mesaon the intrinsic HBT portion and the base contact on the base contactportion. The metal layer forms a base contact 131, a connecting bridge132 between the intrinsic HBT portion and a contact 133 on the base mesaon the intrinsic HBT portion. Gaps 135 separate the emitter 130 from thecontact 133. It is to be appreciated that a plurality of conductivematerials can be employed to provide the functionality associated withthe base metal. The resultant structure is illustrated in FIG. 7.

A photoresist layer 134 is formed on the base layer 126 as illustratedin FIG. 8. The photoresist layer 134 has a thickness suitable forfunctioning as a mask for etching the underlying base layer 126. Thephotoresist layer 134 is then patterned by a performing selectiveirradiation 210 (FIG. 9) to define a base mesa etch mask. The irradiatedor nonirradiated portions are removed by applying a developer material.Any suitable photolithographic techniques can be performed to form apatterned photoresist material layer. Alternatively, the base mesa etchmask can be formed employing a dielectric hard mask. FIG. 10 illustratesa top view of a portion of the resultant structure including the basecontact 131, the connecting bridge 132, the contact 133, the emitter130, and the base mesa etch mask 134.

An etch 220 (FIG. 11) is performed to form base mesa regions 142, 144and 146 (FIG. 12) from the arsenic based base layer 126. The etch 220can be a wet etch using an acid base wet etchant, such as a mixture ofPhosphoric Acid (H₃PO₄) and Hydrogen Peroxide (H₂O₂). Typically, the wetetch is an isotropic etch that undercuts the base mesa etch mask in alldirection. However, by employing a short etch, the base mesa etch maskwill not undercut substantially. A selective etch technique is used toetch the arsenic based base layer at a relatively greater rate ascompared to patterned photoresist layer 134, the base metal layer 132and the underlying phosphorous based collector layer 124. The collectorlayer 124 acts as a stop layer to the etch 220, so that the etching 220does not gouge into the collector layer 124. The base mesas 142 and 146are formed as a result of the etch 220. A remaining portion 144 of thebase layer 126 remains underneath the bridge 140.

An etch 230 (FIG. 13) is performed to remove the phosphorous basedcollector material to form collector regions 148 and 152 (FIG. 14). Theetch 230 employs the base mesa regions as a mask. The etch 230 is a wetetch using an acid base wet etchant, such as an HCl wet etch. Aspreviously discussed, phosphorous based materials have unusual etchingproperties. The collector etch only undercuts facets that are bothoriented at about a +/−45° or about a +/−135° to a major flat of thewafer and that have convex (i.e., greater or equal to 180°) geometricsurfaces. Therefore, during the etch 230, the collector material isremoved underneath the bridge, since the bridge is oriented at about a+/−45° or about a +/−135° to a major flat of the wafer, and has a sidethat is convex. The etch 230 forms both the collector regions 148 and152 and an open region 150 (FIG. 14) under the remaining base material144 and the connecting bridge 132.

FIG. 15 illustrates a partially complete HBT device after a strippingstep (e.g., ashing in an O₂ plasma) is substantially complete to removeremaining portions of the patterned photoresist layer 134. A secondphotoresist layer 136 is formed on the resultant structure and patternedto form a subcollector mask. Next, an etch 240 (FIG. 16) is performed toremove the phosphorous based subcollector material to form subcollectorregions 154 and 156 (FIG. 17). The etch 240 is a wet etch using an acidbase wet etchant, such as an HCl wet etch. As previously discussed,phosphorous based materials have unusual etching properties. If aphosphorous based subcollector is employed, the subcollector etch onlyundercuts facets that are both oriented at about a +/−45° or about a+/−135° to a major flat of the wafer and that are convex (i.e., greateror equal to 180°). Therefore, during the etch 240, the subcollectormaterial is removed underneath the remaining base material 144 and theconnecting bridge 132, since the connecting bridge 132 is oriented atabout a +/−45° or about a +/−135° to a major flat of the wafer, and hasa side that is convex. The etch 240 forms both the subcollector regions154 and 156 and extends the open region 150 under the base material 144and the bridge 132 to the InP substrate 120.

FIG. 18 illustrates a top view of the resultant HBT structure. Anadditional process is performed to provide a collector contact 158,which is coupled to the subcollector region 156 completing the formationof the HBT structure. The HBT structure includes the base contact 131,the connecting bridge 132, the base 133, the emitter 130, and thesubcollector region 156. The first collector mesa 148 and the secondcollector mesa 152 are illustrated with dashed lines. The first basemesa 142 resides under the base contact 131, the second base mesa 146resides under the base 133, and the remaining base material 144 residesunder the conducting bridge 132. As illustrated in FIG. 18, the HBTdevice has a base undercut cantilever base contact structure thatfurther reduces the base-collector capacitance. The result is provideddue alignment of the orientation of the intrinsic HBT base contact alongabout a +/−45° or about a +/−135° angle to the major flat. Therefore,undercutting portions of the collector and subcollector that have aconvex geometric surface during the etching process.

What has been described above includes exemplary implementations of thepresent invention. It is, of course, not possible to describe everyconceivable combination of components or methodologies for purposes ofdescribing the present invention, but one of ordinary skill in the artwill recognize that many further combinations and permutations of thepresent invention are possible. Accordingly, the present invention isintended to embrace all such alterations, modifications and variationsthat fall within the spirit and scope of the appended claims.

1. A heterojunction bipolar transistor (HBT) device comprising: anintrinsic HBT portion formed from a stacked layer having a subcollectorregion, a collector region disposed above the subcollector region, abase mesa disposed above the collector region and an emitter disposedabove the base mesa; a base contact portion having a base contactdisposed away from the intrinsic HBT portion; and a conductive bridgethat connects the base mesa with the base contact, the conductive bridgeoriented at about one of a 001, 010, 00{overscore (1)}, and 0{overscore(1)}0 direction with respect to a major flat of the wafer, theconductive bridge exposing convex edges along at least a portion of oneside of the HBT device.
 2. The HBT device of claim 1, the collectorregion formed from a phosphorous based material.
 3. The HBT device ofclaim 2, the phosphorous based material being one of Indium Phosphide(InP) and Indium Gallium Phosphide (InGaP).
 4. The HBT device of claim2, the base mesa formed from an arsenic based material.
 5. The HBTdevice of claim 4, the arsenic based material being one of IndiumGallium Arsenide (InGaAs), Indium Aluminum Arsenide (InAlAs), IndiumGallium Aluminum Arsenide (InAlGaAs), Gallium Arsenide Antimonide(GaAsSb) and Gallium Arsenide (GaAs).
 6. The HBT device of claim 1, thebase contact portion formed from a stacked layer having a subcollectorregion, a collector region disposed above the subcollector region, abase mesa disposed above the collector region and the base contactdisposed above the base mesa.
 7. The HBT device of claim 1, furthercomprising an opening below the conductive bridge between the collectorregions of the base contact portion and the intrinsic HBT portion. 8.The HBT device of claim 1, the opening extends between the subcollectorregions of the base contact portion and the intrinsic HBT portion belowthe conductive bridge.
 9. An integrated circuit comprising the HBTdevice of claim
 1. 10. The HBT device of claim 1, the conductive bridgeoriented at about one of 001, 010, 00{overscore (1)}, and 0{overscore(1)}0 direction with respect to a major flat of a wafer duringfabrication of the HBT device.
 11. A heterojunction bipolar transistor(HBT) device fabricated on a semiconductor wafer having a major flat,the HBT device comprising: an intrinsic HBT portion formed from astacked layer having a subcollector region, a phosphorous basedcollector region disposed above the subcollector region, an arsenicbased base mesa disposed above the phosphorous based collector regionand an emitter disposed above the arsenic based base mesa; a basecontact portion having a base contact disposed away from the intrinsicHBT portion; and a connecting bridge that electrical connects the basemesa with the base contact, the connecting bridge oriented at about oneof 001, 010, 00{overscore (1)}, and 0{overscore (1)}0 direction withrespect to the major flat of the wafer during fabrication.
 12. The HBTdevice of claim 11, the intrinsic HBT portion, the base contact portionand the connecting bridge aligned at about one of 001, 010, 00{overscore(1)}, and 0{overscore (1)}0 direction with respect to the major flat ofthe wafer during fabrication.
 13. The HBT device of claim 11, theconnecting bridge having at least one side with a convex geometricsurface, such that the connecting bridge is undercut during at least oneof a collector etch and a subcollector etch.
 14. The HBT device of claim11, the phosphorous based material being one of Indium Phosphide (InP)and Indium Gallium Phosphide (InGaP).
 15. The HBT device of claim 14,the arsenic based material being one of Indium Gallium Arsenide(InGaAs), Indium Aluminum Arsenide (InAlAs), Indium Gallium AluminumArsenide (InAlGaAs), Gallium Arsenide Antimonide (GaAsSb) and GalliumArsenide (GaAs).
 16. The HBT device of claim 15, the subcollector formedfrom one of a phosphorous based material or an arsenic based material.17. The HBT device of claim 11, the base contact portion formed from astacked layer having a subcollector region, a collector region disposedabove the subcollector region, a base mesa disposed above the collectorregion and the base contact disposed above the base mesa and an openingbelow the connecting bridge between the collector regions of the basecontact portion and the intrinsic HBT portion, the opening formed duringat least one of a collector etch and a subcollector etch that undercutsthe connecting bridge.
 18. The HBT device of claim 11, the intrinsic HBTportion having a cantilever base contact structure. 19-27. (canceled)